750 barrel and clyinder head troubles

Hello,

Got a problem with my 750 cylinder barrel and cylinder head.

The holes in the cylinder barrel (where the 3 bolts from the cylinder head go down through ....#2, #7 and #8 in the tightening sequence….1.7/8" x 3/8" 26TPI BSC/CEI 20TPI BSF bolts…) are enlarged to 12mm in diameter and have a chamfer about 13-14mm in diameter. Especialy #7 and #8.

My threads for the bolts #7 and #8 got stripped out of the cylinder head…and me thinking it is because of the holes in the cylinder barrel that are too large.

Here are my questions…hope you can help me:

• Do you also think that the large holes are the culprit for pulling my threads out of the head?
• My idea to fix this problem would be: closing the holes with a plug (steel-rod app. 14mm OD with outer thread ...machined into barrel) and drill new holes with right diameter into the plugs.
• What do you think? Got any hints for me for this job?
• What is the right diamter to drill?

Greeting from (cold as hell) Austria, Chris

The chamfer sounds more of an issue than the oversized holes to me
Do you have any photos?
Have the holes in the cylinder head also been opened up?

Hi Chris
Although I can't see the merit in enlarging these holes I really don't think they will be the culprit of your pulled threads. That will, I believe be down to two things (and/or):
1. someone has over-torqued those fasteners (and probably others) in the past. The ones in cast iron just don't give up - they squash the head instead.
2. the lack of forgiveness in the standard 3/8 bolts/studs. The waisted versions available allow a bit of springy "give" when the alloy head expands with temperature.

Which leads to a possible #3. excessive time idling. The head gets very hot and weaker (with temperature) potentially contributing to your woes.

A cheaper and easier version of your fix would be to drill and shrink in small 1/2" long inserts in the top (or top & bottom) of these holes. I'm sure they will be just as effective (and easier) as what you're proposing.
Cheers
Rob

Chris Zet said:

3 bolts

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Studs.

Chris Zet said:

1.7/8" x 3/8" 26TPI BSC/CEI 20TPI BSF bolts…

Click to expand...

According to AN the number two nut is Whit Form so the stud threads should also be 55 degree Whit Form not 60 degree BSC.
Hopefully one day AN will answer this.

Rinse and repeat

The enlarged stud through holes in the barrels should have nothing to do with the stripped stud threads in the head. More likely somebody tried to tighten the nuts on those studs when the head was hot and pulled the studs. Enlarging and chamfering the 3 holes in the barrels would make it easier to drop a stubborn head down on the barrels.

I have a Commando head that doesn't like to be dropped down on a set of pre-Commando 750 barrels. The stud holes or studs just don't get along. It does eventually go down on the barrels but it takes a lot of jostling, and a ridiculous amount of time. The stock pre-Commando head drops right on. Studs are the same size on both heads, and the pre-Commando barrels have been modified to use 3/8" bolts either side of the spark plugs.

Pulling out those studs is a common problem. See the solution here:

jseng1 said:

Pulling out those studs is a common problem. See the solution here:

Click to expand...

The problem with the studs pulling out is common on Norton heads. A design fault from the factory as a fine thread is not recommended in aluminum casting. The above solution addresses this by using 3/8-16 studs.